Private contractors used excessive restraint on low-risk asylum seekers on a removal flight out of the UK, inspectors have revealed in a damning report.

Escort staff were led to believe by dire warnings during a staff briefing that they were dealing with a high-risk group, when the majority of passengers had no history of being disruptive, Her Majesty’s Inspectorate of Prisons (HMIP) said in a report.

The staff, who outnumbered detainees by more than three to one and were employed by Tascor, which is part of the outsourcing giant, Capita, used disrespectful language about detainees, including referring to one as “an arse”, inspectors said.

Some staff were flippant and spoke to each other in “loud and jocular” voices, the report said, while inspectors were concerned that the only female detainee on the flight was forced to use the toilet with the door open in an “unnecessary intrusion into privacy”.

All but one of the 23 detainees on the flight out of London Biggin Hill airport in January were placed in waist restraints, which restrict arm movements and can hold arms clamped to the body in the secure position, HMIP said. Inspectors branded the use of the restraints as “not necessary, proportionate or reasonable”.

The chief inspector of prisons, Peter Clarke, said: “We regularly inspect other detention settings where far more disruptive and challenging behaviour is managed without such physical restraints.

“Clearly, some senior-level intervention is required to ensure that the situation is rectified without delay.”

HMIP’s report is published at a time when the department’s immigration procedures are under intense scrutiny following the Windrush scandal and controversy over removal targets. Another outsourcing giant, Mitie, will be taking over the contract from May.

The inspection covered the removal of 23 people from two immigration removal centres – Colnbrook and Brook House – by 80 staff on a charter flight from the UK to France, Austria and Bulgaria.

The asylum seekers were being removed under an EU convention that requires them to return to the first “safe” country they had entered for a final decision. The removals are managed by the Home Office.

In its report, HMIP said a staff briefing prior to the flight had “emphasised the risks of disruption and the need for vigilance”, focusing solely on control and not on detainees’ treatment or welfare.

One manager said: “Tonight we don’t mess around. If you do, you may well get hurt.”

“Staff clearly thought they were dealing with a very high-risk group. However, the dire warnings they were given were not grounded in evidence.”

Home Office notes on detainees from Brook House, near Gatwick, stated that most had no history of being disruptive and presented “minor or no risk to successful transfer”.

A Home Office spokesman said: “The dignity and welfare of all those in our care is of the utmost importance and we are taking the concerns raised by the inspector in this troubling report extremely seriously.

“Action is already under way to address his recommendations. This includes the introduction of body-worn cameras under the new escorting contract with Mitie, which commenced on 1 May.

“Any use of force must be justified and proportionate and used only as a last resort. We are working with Mitie to ensure that sufficient focus is placed on welfare considerations.”

Capita said it could not comment on individual cases, but any complaints received would have been investigated thoroughly.